In studying the effectiveness of insulation in dielectrics or electrical phenomenon such as lightening, it is important to acquire information as to the phenomenon which occur just prior to the electrical discharge. Such information is of enormous value to industries such as the aircraft industry, which is currently utilizing and exploring further utilization of high-strength, light weight materials which are non-metallic. It is very unusual for lightening strikes on an airplane having a metallic skin to cause damage because the electrons are simply conducted from the part of impact over the metallic skin and back into the atmosphere from a discharge point. If the aircraft is made of a dielectric material, there is the distinct possibility that the aircraft will no longer be protected from lightening strikes, thus severely limiting or perhaps negating altogether further widespread use of dielectric materials as aircraft skins. There are, of course, many other reasons to study electrical breakdown phenomena or partial discharges through insulation so as to improve the design of electrical equipment generally. For example, in the electrical power industry, blown transformers cost utilities and their customers tens of millions of dollars per year. A better understanding of the behavior of the insulating liquids used in large electrical transformers could lead to improved transformer design, resulting in enormous savings to these utilities and their customers.
In electrical breakdown processes in general, it is the pre-breakdown events that are often of interest. These events may last only a few microseconds or less prior to the breakdown. In studying phenomenon such as electric breakdown, it is most helpful to have photographs of the event; however, due to the random nature of electrical discharges even within a very constricted time frame, it is sometimes very difficult to predict exactly when the discharge will occur. Consequently, the approach generally taken is to arrange for the photographic recording device to take pictures approximately when the breakdown is anticipated. After many pictures one may be able to capture the phenomena of interest. This is difficult if the phenomena are fast and the time between occurrences is long. Alternatively, it is possible to operate a trigger upon the occurrence of the discharge and to operate an image-converter camera or other highspeed photographic device with the trigger at the instant of discharge. The resulting photograph does not include what happened just prior to discharge, and thus a complete visual understanding of electrical discharge phenomenon is extremely difficult to precisely obtain. Since the time of a random discharge cannot be known precisely over time intervals which are long compared to the camera recording time, it is extremely difficult to plan for photographing the events which occur a few microseconds or less prior to dielectric breakdown. Accordingly, there is a need for a method of optically recording these prior phenomena and a need for an apparatus to practice the method.
Though there are inventions involving optical delay paths, none have the following features basic to the present invention: a spherical focusing mirror facing an array of flat mirrors arranged to preserve a well-focused image by repeatedly refocusing and correcting for asigmatism; recording triggered by a random event and recording only the images of interest; and recording images of conditions that occurred before recording was triggered.
U.S. Pat. No. 4,547,787, Kaneko, discloses an image-forming apparatus with displaying and printing functions. Unlike the present invention, images are continually recorded as a negative and printed as a positive. Thus, everything must be recorded. There is no optical delay path that uses only the repeated reflection and refocusing of a light beam to achieve an optical delay.
U.S. Pat. No. 3,025,406, Stewart et al., discloses a light screen for ballistic uses. Light is reflected back and forth between two screens, but the reflections are not intended to refocus an image and recording is not triggered by a random event.
U.S. Pat. No. 3,437,954, Herriott et al., discloses an optical delay line device with a pair of curved mirrors that form a resonator. A well-focused image is not produced, nor is recording triggered by a random event.
U.S. Pat. No. 3,571,738, Gloge, discloses a folded optical delay path. A well-focused image is not produced, nor is recording triggered by a random event.
U.S. Pat. No. 4,626,078, Chernin et al., discloses a multiple pass optical matrix system. A well-focused image is not produced, nor is recording triggered by a random event.